1. Field of the Invention
The present invention relates to a central processing unit with means for easily initializing an internal register.
2. Description of the Related Art
In a general central processing unit, a vector address is generated in a reset sequence and only address data showing the location of an instruction to be executed next after a resetting operation is fetched from an external data table. The vector address is also generated in an interruption sequence which is not a resetting operation after the operating state of an internal register is stored to an external stack, etc. Then, only address data showing the location of an instruction to be executed next is fetched from the external data table.
Accordingly, in such a general central processing unit, only address data showing the location of the next instruction is referred to the data table by outputting the vector address in the reset sequence. Therefore, the internal register within the central processing unit is initialized at the resetting time by one of the following two methods.
(1) A method for initializing the internal register in a program by using software means such as an instruction for loading data after the completion of the reset sequence.
(2) A method for performing an initializing operation by arranging dedicated hardware for initializing a plurality of internal registers to constant values and controlling the operation of a circuit added to the dedicated hardware at the resetting time.
In method (1), since software means is used, it is necessary to make a plurality of programs in the initializing operation after the reset sequence in a series of resetting operations (including the initializing operation). Therefore, programming size is increased and the time for executing the series of resetting operations is increased.
In method (2), it is possible to perform the initializing operation at a high speed in comparison with method (1). However, the circuit size is large since the dedicated hardware must be additionally disposed. Further, since the initialized data are fixed to a constant value, software means such as that in method (1) must be used when this value is changed to an arbitrary value.